A W126 is a heavy car with a relatively narrow tire. This, in theory, makes a W126 more resistant to hydroplaning than a lighter vehicle equipped with a wider tire. I would wonder:
1. Are the current tires too old and hardened? You can forget about optimal traction in any condition under this circumstance, even if the tire is a premium brand.
2. Are your current rear tires worn excessively on the inside? I notice a fair number of 420's on the road with excessive negative camber of the rear wheels (bottom of tires stick out further than the top, as if you had 2,000 lbs. in the trunk). This was the hot setup on 1930's Auto Union Grand Prix racers, but has no place on the street. If this condition exists on your car, search this site for discussion of the problem/solution.
3. Oliverp discusses significant control problems in the wet with his 300SE. If I didn't know what make of car was involved, I would have assumed he was driving a late '60's Corvette with 400 hp and a limited slip differential. The behavior he is experiencing on a well balanced 4,000 lb. car with a six cylinder engine and open differential is very odd. I can't recall if the w126 has a rear subframe. If it does, I would investigate the possibility of worn components allowing the subframe to shift position under power or braking, causing the rear wheels to change direction and steer the vehicle.